Various terms are defined in the following specification. For convenience, a Glossary of terms is provided herein, immediately preceding the claims.
Frequently there is a need for marine structures that have high strength. For example, offshore platforms for production of hydrocarbons, such as compliant pile towers and deep draft caisson vessels, must have adequate strength to support processing equipment of substantial weight. Similarly, the processing equipment must have adequate strength to contain the hydrocarbons and/or other fluids used during the processing. Other marine structures that require strength, to name a few, are ship hulls, submarine hulls, marine vessel mooring chains, caissons, buoys, bridges, subsea transport lines for fluids, dams, seawalls, and retaining walls.
Commercially available carbon steels with low to moderate strength (35-50 ksi yield strength) are traditionally used to construct marine structures. In certain cases, commercially available nickel-containing steels (3½ wt % Ni to 9 wt % Ni), aluminum (Al-5083 or Al-5085), or stainless steel are used. Also, specialty materials such as titanium alloys and special epoxy-impregnated woven fiberglass composites are sometimes used. However, marine structures constructed from these materials often have increased wall thicknesses to provide the required strength. This adds weight to the marine structures that must be supported and/or transported, often at significant added cost to a project. The added cost for support and transport of the marine structures combined with the increased cost of the material for construction tends to decrease the economic attractiveness of projects.
Many marine structures must also have adequate fracture toughness to resist fracture initiation and cracking. Arctic environments, to which marine structures are often subjected, tend to make obtaining construction materials having the required combinations of strength and fracture toughness more difficult. For example, an offshore platform for production of hydrocarbons may be subject to arctic environments, i.e., temperatures down to −40° C. (−40° F.).
Nickel-containing steels conventionally used for structural applications that may be subjected to cold temperatures, e.g., steels with nickel contents of greater than about 3 wt %, have good fracture toughness, but also have relatively low tensile strengths. Typically, commercially available 3.5 wt % Ni, 5.5 wt % Ni, and 9 wt % Ni steels have yield strengths of up to about 275 MPa (40 ksi), 450 MPa (65 ksi), and 620 MPa (90 ksi), respectively, and corresponding tensile strengths of up to about 485 MPa (70 ksi), 620 MPa (90 ksi), and 690 MPa (100 ksi), respectively. In order to achieve combined strength and cold temperature fracture toughness, these steels generally undergo costly processing, e.g., double annealing treatment. In certain applications, industry currently uses these commercial nickel-containing steels because of their good fracture toughness at low temperatures, but must design around their relatively low tensile strengths. The designs generally require excessive steel thicknesses for load-bearing applications. Thus, use of these nickel-containing steels in load-bearing applications tends to be expensive due to the high cost of the steel combined with the steel thicknesses required. Additionally, not all of these steels retain the required cold temperature fracture toughness in the heat affected zone (HAZ) when welded.
A need exists for marine structures suitable for economically producing and processing hydrocarbons for commercial use, particularly such marine structures that have a cost per unit strength substantially lower than that of currently available marine structures. In some cases, a need exists for such marine structures that also have adequate fracture toughness in both the base plate and the HAZ for use in arctic environments, i.e., at temperatures down to −40° C. (−40° F.). Additionally, other such marine structures suitable for use in arctic environments are needed.
Consequently, an object of this invention is to provide marine structures suitable for economically producing and processing hydrocarbons for commercial use. Another object of the present invention is to provide such marine structures having a cost per unit strength substantially lower than that of currently available marine structures. A further object of this invention is to provide such marine structures having the required combinations of strength and fracture toughness for use in arctic environments. Other objects will be made apparent by the following description of the invention.